The present invention relates to a manufacturing method and device for a polycrystalline silicon, and more particularly to a polycrystalline silicon (p-Si) which poly-crystallizes a thin film amorphous silicon (a-Si) using a laser.
Generally, p-Si is in wide use for a direct random access memory (DRAM) semiconductor element or for a liquid crystal display (LCD) wherein a polycrystalline silicon thin film transistor (TFT) is used or for a contact image sensor (CIS). As the conventional method for manufacturing p-Si, the solid phase crystallization (SPC) method performs a heat treatment on a-Si or on a micro-crystalline silicon (.mu.-Si) at a high temperature, i.e., nearly 1000.degree. C., and the excimer laser annealing (ELA) method scans the surface of a-Si or .mu.-Si with an excimer laser which then is melted and crystallized.
U.S. Pat. Nos. 4,851,363 and 4,880,753, Japanese Patent Laid-open Publication Nos. sho 62-104117, sho 63-119576, sho 64-025515, hei 02-033935 and hei 02-143559 and GB 2,169,442 disclose the above manufacturing method in various types.
The SPC method enables manufacturing p-Si material whose characteristics are regular. However, a low-cost glass substrate cannot be used for the SPC method since the material, i.e., a-Si, is preformed by heat treatment at a high temperature, which means that the substrate used for the SPC method is greatly restricted.
The low heat treatment method being performed in an approximate 600.degree. C. environment is proposed by relevant publications (Optronics, No.8, 1991, pp 53, and IEEE Electron Device Letters, Vol. 10, No.8, Aug. 1989, pp 349). However, this method also does not permit the use of the low-cost glass substrate.
To improve this problem, the ELA method which performs heat treatment using an excimer laser having a high optical absorption for a-Si is applied. Since the excimer laser used for ELA method forms a short recurring pulse of short wavelength, most of the optical energy is absorbed in the a-Si surface. As a result, a small amount of heat is delivered to the substrate, to thereby enable the advantageous annealing condition which does not give a thermal transformation on the substrate. Additionally, a device whose operation speed is excellent, for example, TFT or TFT LCD, can be manufactured according to the ELA method. However, the pulse-to-pulse energy stability (PPES) of a laser is poor since the duration of the laser pulse used for ELA method is no more than 10-20 ns. It is hard to evenly perform heat treatment throughout the entire surface of substrate due to the characteristic degradation of the scanned region where the beam overlaps. That is, the laser energy absorbed in a-Si is converted into a thermal energy at the moment when an excimer laser beam reaches the a-Si surface, as shown in FIGS. 1 and 2. Accordingly, a-Si is re-crystallized by the thermal energy, and the melting and re-crystallization processes take a very short time, i.e., approximately 100-500 ns. Since the necessary time for the crystallization is very short, the grain size of the polycrystalline silicon which is crystallized according to the ELA method is no more than a matter of hundreds of angstroms. In addition to this, the crystallization state according to the scanned region where the laser beam is irradiated by being overlapped and with respect to the deviation of the laser energy output, becomes serious since the p-Si characteristics change sensitively depending on the change of the energy density.
To solve this problem, a method by which the ELA method is performed while heating the substrate at the same time was proposed in Solid State Devices Materials (1991). This disclosure reveals that the grain size of the crystal increases when the crystallization speed is slowed by performing the ELA operation while heating the substrate, and the scanned region where the laser beam overlaps can be crystallized stably.
However, the substrate cannot be heated sufficiently since the temperature to which the substrate may be heated is limited, and excessive heating time is needed, which results in lowering productivity.